1. Field of the Invention
The present invention is directed to a method and vacuum plasma chamber for improving the treatment rate of a plasma enhanced vacuum treatment of a surface of workpiece.
We thereby define the treatment rate of such vacuum treatment of a surface as the amount of material per time unit which is removed or which is deposited from or on that surface respectively.
Thereby the present invention is especially directed on such an improvement for reactive plasma etching, reactive plasma sputter coating, reactive ion plating or and especially for plasma enhanced chemical vapor deposition coating known as PECVD.
Especially for semi-conductor production where plasma enhanced treating processes are used, thereby especially plasma enhanced coating processes, it is of tremendous importance to prevent that the workpiece surface being treated is contaminated by contamination particles, especially by dust or powder particles. Under plasma enhanced coating treatments we especially understand plasma enhanced reactive coating treatments as e.g. plasma enhanced chemical vapor deposition known as PECVD treatments, and thereby especially enhanced by RF plasma at frequencies from 1 to 100 MHz.
Preventing such contamination is a most serious problem to be resolved for such manufacturing processes.
2. Description of Prior Art
The attempts to resolve this problem up to now were directed on trying to minimize the generation of dust or powder during such treatment processes. Nevertheless, such dust or powder generation may not completely be prevented. Therefore, the attempts were additionally directed on removing the residually generated dust as efficiently as possible from that plasma discharge area which is significantly contributing to the treatment. This means that one tried to reach in the plasma discharge space there, where the plasma discharge is significantly contributing to the treatment, a dust- or powder-free state. Reference is made to the EP-A-425 419 and the EP-A-453 780. Attention is further drawn to the following prior art: EP-A-0 419 930 (corresponding to JP-A-3 153 885 and U.S. Pat. No. 5,102,496), EP-A-0 453 780 (corresponding to JP-A-5 074 737), EP-A-0 425 419 (corresponding to CA-A-2 024 637 and JP-A-3 147 317), EP-A-0 272 140 (corresponding to U.S. Pat. No. 5,000,113, JP-A-63 246 829, U.S. Pat. Nos. 4,872,947, 4,892,753, 4,960,488, 5,158,644).
It is an object of the present invention to improve the treatment rate and thereby especially, for a plasma enhanced coating deposition vacuum process, the coating rate without negatively affecting surface quality of the surface being treated, but thereby even improving its quality.
This object is resolved by a method for improving the treatment rate of a plasma enhanced vacuum treatment of a surface of a workpiece which comprises the step of generating substantially along the surface to be treated in the plasma dust with a predetermined distribution of its density.
The basis of the present invention is the recognition made by the inventors that dust or powder particles in a plasma discharge significantly increase the coupling degree of electrical energy to the plasma. Due to this phenomenon the treatment rate and especially for a layer deposition treatment, the deposition rate is significantly risen. Additionally, such dust or powder particles in the plasma discharge lead to improved coating quality as e.g. to improved characteristics of the tensions within the deposited film or layer and of its fineness. This, nevertheless, is only true as long as it may be prevented that powder or dust particles accumulate on the surface being treated.
A typical vacuum range of operating pressure is between 10xe2x88x922 mbar and 10 mbar, thereby preferably between 10xe2x88x921 mbar and 1 mbar.
Although the object to be fulfilled will also be resolved e.g. for reactive sputter etching treatments, it shall especially be resolved for coating processes, thereby especially for reactive plasma enhanced processes and especially RF plasma enhanced reactive coating processes, so-called RF-PECVD processes.
Although especially directed on the RF-PECVD treatments, the inventive method may principally also be used for DC or AC plasma treatment or for hybrid forms with AC+DC plasma.
In contrary to the customary approaches, namely to remove powder or dust as completely as possible from the plasma discharge area which is affecting the surface treatment, the present invention maintains in a well controlled manner the powder trapped in the plasma discharge so as to reach the advantages with respect to treatment rate and treatment quality mentioned above. Additionally, the density of the dust or powder entrapped in the plasma discharge is maintained below a predetermined value at which value powder or dust deposition on the surface being treated would start.
Thus, inventively, the number of dust particles per volume unit and/or the largeness of dust particles and thus generally the dust density and its distribution is controlled especially not to vanish. By such control predetermined density and density distribution are achieved which have been found as optimal for a specific treatment process considered during pre-experiments. Thus, prevailing dust is exploited and not just removed as completely as possible.
It is a second object of the present invention to control dust density in a plasma discharge. This is reached by a method for reducing dust density in a plasma discharge space with a plasma to which a workpiece surface to be treated is substantially uniformly exposed, which comprises the step of applying substantially parallel to the surface to be treated and across said surface in said plasma a field of force acting on dust particles being trapped in said plasma.
By generating such a field of forces and by adjusting such field as concerns its local extent, its distribution and strength, a desired amount of powder or dust is removed from that plasma discharge space which is significantly contributing to the surface treatment. The removed powder or dust is primarily conveyed into a space which is less significantly affecting. the treatment and from that space the dust or powder may further be removed whenever necessary.
As a preferred embodiment, there is thus proposed a method for improving the treatment rate of a plasma enhanced vacuum treatment of a surface of a workpiece, which comprises the steps of generating substantially along the surface to be treated. in the plasma, dust with a predetermined distribution of its density and generating said predetermined density by applying substantially parallel to the surface and transversally therealong within the plasma a field of force acting on the particles of the dust, predominant parts thereof being trapped in the plasma.
It is a further object of the invention to control dust density as simply as possible.
This is realized by generating the field of force, at least in parts, in a preferred mode predominantly by generating a gradient of pressure. Thereby, such field of force may also, additionally to the gradient of pressure or exclusively, be generated by an electrostatic and/or by a thermal gradient, the latter in the sense of exploiting thermophoresis.
Further, the primary object mentioned above is resolved by a vacuum plasma chamber with generating means for a three-dimensionally distributed plasma discharge in a discharge space, with a workpiece support defining a support area for at least one workpiece to be treated and for exposing at least one workpiece surface area to said plasma discharge in said space which comprises force generating means generating a field of force substantially parallel to and along said support area and within said plasma discharge, and controlling the density of dust in said plasma discharge.
The primary object is resolved primarily for treatment of substantially flat large areal workpieces, as especially for flat active panel displays, which is achieved by the inventive method and plasma chamber being applied and construed respectively to and for such workpieces respectively.